There is interest on the part of physicians to visualize tissue and body structures in other waveband regimes beyond that of visible bands, which are generally considered to be in or near the range from 400 nm to 700 nm. The near infrared regions (700 nm to 900 nm) where dyes such as Indocyanine green (ICG) fluoresce and are used as a marker, and where tissue has added transparency, are of particular interest. Additionally, the resolution requirements of endoscopes have increased with the introduction of High Definition (HD) detectors used in video imaging owing to smaller and more numerous pixels than earlier NTSC or PAL formats. Accordingly, optical improvements for endoscopes enabling extended waveband performance, higher sampling frequencies, and depth penetration in the near infrared in addition to the normal visible region would be advantageous.
In both diagnostic and therapeutic procedures where endoscopes are used it is advantageous to provide guiding imagery and fluorescent markers. These are used in surgical procedures to extend the physician's visualization of tissues and structures to regions beyond his native capabilities. Accordingly, good visualization in the visible bands plus longer wave length regions outside of human vision are a desirable outcome.
It is well known that detectors made of silicon respond very well in the infrared region, although this capability is normally not used in medical imaging systems as the human eye does not see where silicon responds best, i.e. above 700 nm. As a consequence, optical instruments for surgery have more than ignored this detector capability as it is outside of human vision, but purposefully block it because NIR detector response doesn't correspond to a primary color component of vision, i.e. red, green and blue. Normal imaging systems and the silicon response is binned by primary colors, either by color filters over individual pixels in the case of a single detector or larger color filters used to divide the entire beam path into red, green and blue paths for their own detectors. This later configuration is usually called a three-chip camera. A NIR detector response can be thought of as a black and white response. The challenge is to make this correspond to the optical path length of the visible regime.
Endoscopes are constructed to see deep within the body through a narrow opening or path and consist of numerous optical elements that can be grouped by functional requirements.
Rod lens assemblies or relay lenses are used in grouped pairs such as 3 pairs or 5 pairs and are required to re-image the product of the endoscope objective lens assembly to form an image at the field stop of the endoscope. An ocular or coupling lens for video use then views the combination of field stop and image. The longitudinal chromatic aberrations of existing endoscope relays, when used outside of visible wavebands, add significantly to a displacement of focal points, by wavelength, with longer wavelengths falling substantially behind that of the visible images focal points. This outside of visible waveband displacement error is cumulative, the sum of which is too large for the physician's expectation of, and need for, high resolution. This is particularly troublesome when attempting to resolve fine detail in vascular imaging, nerve imaging, and/or tumor margins in the near infrared spectrum with endoscopes designed for use in the visible spectrum.
Larger diameter endoscopes generally have faster f-numbers than smaller diameter endoscopes. More optical performance is required of larger diameter endoscopes due to physician expectations that larger endoscopes have high resolution and the nature of optical correction that requires more effort be applied to correcting faster systems as the optical train is enlarged. The diameter to length ratio of a 10 mm endoscope is larger than that of a 2.8 or 4 mm diameter endoscope, resulting in improved brightness and more resolution. 10 mm diameter endoscopes, and endoscopes of similar diameter, have relay systems that operate at approximately 1:6 (F 6) while 4 mm diameter endoscopes have relays that operate at approximately 1:7 (F 7), with smaller endoscopes operating at a correspondingly lower F-number.
U.S. Pat. No. 3,257,902 to Hopkins taught that filling the air spaces within an endoscope relay with glass rods substantially increased the operating F-number of an endoscope over that of existing endoscopes, which used cemented doublets widely spaced to produce a telecentric relay system. This resulted in brighter images and better diagnosis in visible wavebands.
It had been well known that a telecentric design was a requirement for producing a relay system without substantial vignetting. T. H. Tomkinson, J. L. Bentley, M. K. Crawford, C. J. Harkrider, D. T. Moore, and J. L. Rouke, “Rigid endoscopic relay systems: a comparative study,” Appl. Opt. 35, 6674-6683 (1996).
To produce a non-vignetting telecentric relay at a fast F-number has been taught in U.S. Pat. No. 5,005,960 to Heimbeck and U.S. Pat. No. 5,684,629 to Leiner. These references show the production of well-corrected images in the center of the field in visible wavebands, though in most cases it is an achromatic correction due to the limited number of degrees of freedom. Residual aberrations such as astigmatism and coma are typically corrected for off axis field points in endoscopes by use of offsetting aberrations in the objective lens assembly. In particular, it is well known that field curvature is produced by positively curved elements, and in almost all cases the relay systems of endoscopes are composed of positively powered surfaces at their respective air-glass interfaces on each end of the rod assembly. There is a large amount of field curvature to be compensated for in the objective, which is successfully done for visible wavebands in present endoscopes.
When one adds the demand of high correction in a waveband outside of the visible (e.g., near infrared correction) it becomes necessary to add more degrees of freedom to the relays in the form of additional lens elements of differing materials. Endoscopic relays with 2 materials such as U.S. Pat. No. 5,005,960 provide fine achromatic correction on axis, but at 0.7 field and full field points there is significant uncorrected astigmatism, field curvature, coma and chromatic aberrations consistent with 2 element solutions, commonly called achromatic correction. Therefore 3 and 4 material solutions must be considered for super apochromatic performance, i.e., correction at 4 wavelengths, RGB in the visible and NIR (near infrared).
Additionally, market needs have changed with the introduction of high definition video. Where once manufacturers sought to lower parts count to reduce cost, which favored a limited number of elements, high definition video (HD) now requires higher performance so the number of elements must be rethought. By judicious use of radii and glass selection, additional glass elements can be added to the relay systems to take advantage of manufacturing efficiencies that are gained by the selection of longer radii because more of such lenses can be added to a fabricating tool by the opticians. With short radii one, or a few, surfaces are produced on a grinding or polishing tool at a time. With longer radii a larger number can be produced in the same grinding or polishing time. Additionally, more lenses per tool yield more accurate results. Cemented components are more likely to be aligned properly than individual glass elements assembled in combination with metal spacers, so added elements should be considered as part of a bonded assembly.
Accordingly, there is a well-defined and long-felt need in the marketplace for an HD endoscope that can resolve both visible and near infrared light in approximately the same plane and, thus, can enhance visibility of tissues and structures for surgeons.